The simplest way to find out if the fault is in the DVD drive itself, or in the Windows installation, is to try to boot from a bootable DVD eg Windows installation DVD or Linux Live DVD like many computer magazine cover disks. If the drive cannot recognise a DVD to boot from, the drive itself is the problem. A lens clean is the only thing you could then try, otherwise the DVD section of the laser is normally what has failed, and the drive would need to be replaced.
If booting from the DVD works, the drive is reading OK, we can then proceed with fault-finding the Windows installation. But there's no point spending time doing that if the drive itself has failed, the commonest cause of this symptom.
Edited by Platypus, 25 June 2008 - 05:44 AM.